Sheet conveying apparatus

Abstract: The present invention relates to a sheet conveying apparatus wherein a pair convey rollers (6) arranged upstream of a registration means (8) for correcting a skew-feed of a sheet in a sheet conveying direction can be abutted against or separated from each other, or can be shifted in accordance with a length of the sheet in the sheet conveying direction. As the result, when the skew-feed of the sheet is corrected by the registration means (8), a trailing end of the sheet is not restrained.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a sheet conveying apparatus according to the preambles of claims 1 and 5. Those sheet conveying apparatuses are used with an image forming apparatus or image reading apparatus such as a copying machine, a scanner, a printer and the like. More particularly, it relates to a sheet conveying apparatus wherein registration of sheet is effected at an upstream side of an image forming portion or an image reading portion. Related Background Art Conventionally, in image forming or image reading apparatuses such as copying machines, scanners, printers and the like, a registration device for determining posture and position of a sheet is arranged immediately in front of an image forming portion or an image reading portion. As such registration devices, for example, there has been proposed a registration device of loop registration type wherein a tip end of a sheet is abutted against a nip between a pair of stopped rollers to form a loop in the sheet, so that the tip end of the sheet is aligned with the nip, thereby correcting skew-feed of the sheet, and a registration device of shutter registration type wherein a shutter member for stopping a tip end of a sheet is retractably arranged in a sheet conveying path. After the tip end of the sheet is abutted against the shutter member to correct skew-feed of the sheet, the shutter member is retracted from the sheet conveying path. However, recently, as such image forming or image reading apparatuses have been changed into digital design, a distance between sheets (referred to as "sheet interval" hereinafter) has been shortened so that a large number of sheets can be treated within a short time period, thereby increasing substantial image forming speed without increasing a process speed for image formation. For example, in conventional digital copying machines, when a copying operation is performed continuously, since an optical system for exposing an original must be reciprocally shifted by times corresponding to the number of copies, the sheet interval used in an image information copying operation is necessarily determined. However, when the original is digitally read and the image is digitally formed, after the original was read once, the image information is electrically coded to be stored in a memory. In the image formation, the image information is read out from the memory, and a latent image corresponding to the image information is formed on a photosensitive member disposed in the image forming portion by using an exposure device including a laser beam or an LED array. Thus, even when a plurality of copies are obtained, mechanical movement of the optical system and the like can be eliminated. Accordingly, the time period for effecting the registration of the sheet is one of important factors for determining the sheet interval. In order to reduce the registration time period, there has been proposed an active registration method for correcting the skew-feed of the sheet while conveying the sheet. In this method, two sensors are arranged in a sheet conveying path and disposed in a plane perpendicular to the sheet conveying path, so that a tip end of the sheet moving in the sheet conveying path is detected by the sensors, respectively, to determine inclination of the sheet on the basis of signals from the sensors, and the skew-feed of the sheet is corrected by controlling sheet convey speeds of skew-feed correction rollers (referred to as "regist rollers" hereinafter) arranged in a plane perpendicular to the sheet conveying path and drive independently. According to this method, since the skew-feed of the sheet can be corrected while conveying the sheet without stopping the sheet temporarily, the sheet interval can be reduced in comparison with other methods. However, in the above-mentioned conventional example, if a size of the sheet to be conveyed is not constant, and particularly, if a long-sized sheet is conveyed, the correction of the skew-feed of the sheet must be effected by the regist rollers under a condition that a trailing end of the sheet is pinched between convey rollers disposed at an upstream side of the regist rollers in the sheet conveying direction. That is to say, in the above-mentioned active registration method, although a slower side (succeeding side) of the sheet is advanced or a faster side (preceding side) of the sheet is delayed by means of the regist rollers, in both cases, since the entire sheet must be rotated, if the trailing end of the sheet is pinched between the convey rollers, it will be hard to rotate the sheet by a desired amount, with the result that it is very difficult to correct the skew-feed of the sheet with high accuracy. In order to eliminate the above drawback, there has been proposed a method wherein a loop is formed in the sheet in front of the regist rollers and the rotational movement of the sheet generated in the skew-feed correction is absorbed by distorting the loop. However, in this method, since the loop is released · as soon as the trailing end of the sheet leaves the convey rollers, if the sheet interval is small, there is a danger of the collision of the trailing end of the sheet against a tip end of a next sheet. Thus, a certain amount of sheet interval or more must be maintained. From the document US-A-4 621 801, a sheet conveying apparatus is known which comprises a registration means for correcting a skew-feed of a sheet and which is arranged at an upstream side of an image forming portion or an image reading portion in a sheet conveying direction, an abuttable/separable sheet convey means arranged at an upstream side of the registration means in the sheet conveying direction and a sheet end detection means which is arranged upstream of the sheet convey means. From the JP-A-04 327 445, a similar sheet conveying apparatus is known which comprises a plurality of convey means for applying conveying forces to a sheet in a simultaneous manner and a skew-feed detection means for detecting a skew-feed of the sheet. Furthermore, it comprises a registration means for correcting a skew-feed of the sheet. From the EP-A-0 485 167, a sheet conveying apparatus is known which also comprises a registration means for correcting a skew-feed of a sheet as well as a sheet convey means arranged at an upstream side of the registration means in the sheet conveying direction. Furthermore, it comprises a sheet skew amount detection means arranged downstream of the registration means in the sheet conveying direction. The registration means are arranged at an upstream side of the image forming portion or the image reading portion in the sheet conveying direction. Furthermore, from the document US-A-5 090 683, a sheet conveying apparatus is known which comprises a registration means for correcting a skew-feed of a sheet, a sheet convey means arranged at an upstream side of the registration means in the sheet coveying direction, a sheet skew amount detection means arranged downstream of the registration means in the sheet conveying direction. Finally, from the document US-A-5 140 166, a sheet conveying apparatus is known which comprises a registration means for correcting a skew-feed of a sheet arranged upstream of an image forming portion or an image reading portion in a sheet conveying direction, an abuttable/separable sheet convey means arranged upstream of the registration means in the sheet conveying direction, a sheet-skew amount detection means and a sheet detection means arranged downstream of the registration means in the sheet conveying direction. In this sheet conveying apparatus, the sheet convey means is initially in an abutted condition and will be separated from each other when a tip end of the sheet is detected by the sheet detection means. SUMMARY OF THE INVENTION The present invention intends to eliminate the above-mentioned conventional drawbacks, and has an object to provide a sheet conveying apparatus which can continuously convey sheets having various sizes with a minimum sheet interval and can effect registration of sheet with high accuracy. Incidentally, when a sheet conveying apparatus is used for a long time, it is supposed that there arises a problem that the rollers are gradually worn to gradually reduce outer diameters of the rollers, thereby worsening the desired convey speeds of the rollers, or paper powder and/or toner is trapped between the rollers to reduce friction forces of surfaces of the rollers, thereby worsening the desired pinching force of the nip between the rollers, and, thus, worsening the desired convey speeds of the rollers. Accordingly, the present invention also aims to eliminate the above problem, and another object of the present invention is to provide a sheet conveying apparatus wherein, even after the apparatus is used for a long time or used repeatedly, registration of a sheet can be effected with high accuracy regardless of the frequency of use of the apparatus. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view of a sheet conveying apparatus according to a first embodiment of the present invention applied to an original image reading apparatus; Figs. 2A and 2B are sectional views showing an abutting and separating mechanism for abutting a pair of convey rollers of the apparatus of Fig. 1 against each other or separating the convey rollers from each other; Fig. 3 is a plan view of an original skew correction portion of the apparatus of Fig. 1; Fig. 4 is a control block diagram of the apparatus of Fig. 1; Fig. 5 is a control flow chart for the apparatus of Fig. 1; Fig. 6 is a control timing chart for the apparatus of Fig. 1; Fig. 7 is a graph showing the change in drive frequency of a step (pulse) motor in reduction control; Fig. 8 is a schematic sectional view of a sheet conveying apparatus according to a second embodiment which is not part of the present invention, applied to an original image reading apparatus; Fig. 9 is a schematic sectional view of a sheet conveying apparatus according to a third embodiment of the present invention applied to an original image reading apparatus; Fig. 10 is a perspective view of a shifting mechanism for shifting a pair of convey rollers of the apparatus of Fig. 9; Figs. 11A and 11B are constructional views showing a sheet length detection means; Fig. 12 is a timing chart showing a signal from the sheet length detection means; Fig. 13 is a schematic sectional view showing the other embodiment of the present invention; Fig. 14 is a control block diagram regarding the embodiment of Fig. 13; and Fig. 15 is a control flow chart regarding the embodiment of Fig. 13. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) Now, a sheet conveying apparatus according to a first embodiment of the present invention which is applied to an original image reading apparatus will be concretely explained with reference to the accompanying drawings. Fig. 1 is a schematic sectional view of a sheet conveying apparatus according to a first embodiment of the present invention applied to an original image reading apparatus, Figs. 2A and 2B are sectional views showing an abutting and separating mechanism for abutting a pair of convey rollers of the apparatus of Fig. 1 against each other or separating the convey rollers from each other, and Fig. 3 is a plan view of an original skew correction portion of the apparatus of Fig. 1. In Fig. 1, a plurality of originals G are rested on an original stacking plate 1 with imaged surfaces thereof facing upwardly. Sensors (sheet length detection means) 2a, 2b serve to detect lengths Ls 1 , Ls 2 (in an original conveying direction) of the originals G rested on the original stacking plate 1, respectively (Ls 1 < Ls 2 ). A pick-up roller 3 has an outer peripheral surface coated by rubber having relatively high friction of coefficient and serves to feed out the original(s) G from the original stacking plate 1. A pair of separation rollers 4 comprises an upper roller 4A and a lower roller 4B which are both rotated in an anti-clockwise direction, thereby separating the originals G fed by the pick-up roller 3 one by one. A first pair of convey rollers 5 and a second pair of separable convey rollers 6 are provided as a sheet convey means. As shown in Figs. 2A and 2B, a roller 6a of the roller pair 6 can be abutted against or separated from the other roller 6b by means of an abutting and separating mechanism 7. In the illustrated embodiment, as shown in Fig. 2A, the abutting and separating mechanism 7 comprises a solenoid 7a which is electrically turned ON/OFF, an arm 7b for connecting the solenoid 7a to the roller 6a, and a spring 7c for urging the roller 6a against the roller 6b. Incidentally, in an initial condition, the solenoid 7a is being energized (ON condition) to separate the roller 6a from the roller 6b. When the solenoid 7a is disenergized, the roller 6a is abutted against the roller 6b by a biasing force of the spring 7c. Further, in Fig. 1, there is provided a registration means 8 for correcting skew-feed of the original G. As shown in Fig. 3, the registration means 8 comprises two sets of rollers (convey means or rotary members) 8a, 8b arranged along a common line transverse to an original conveying direction and driven independently. That is to say, the skew-feed of the original is corrected by increasing or decreasing rotational speed of one set of rollers in accordance with a skew amount of the original so that the convey speed of one set is differentiated from the convey speed of the other set. Incidentally, in the illustrated embodiment, the skew-feed of the original is corrected by decreasing the rotational speed of the paired rollers in one set (regarding a preceding side of the original) in accordance with the skew amount of the original. Further, there is also provided a sheet skew amount detection means 9. As shown in Fig. 3, the sheet skew amount detection means 9 comprises two sensors 9a, 9b of light permeable type arranged along a common line transverse to the original conveying direction and disposed in the original convey path. The skew-feed of the original is corrected by controlling respective drive motors M 1 , M 2 for the pairs of regist rollers 8a, 8b by means of a control means (not shown) on the basis of the skew amounts of the original detected by the sensors 9a, 9b. Further, in Fig. 1, there is also provided an image reading apparatus 10 which comprises an illumination lamp 10a for illuminating the imaged surface of the original G through a transparent glass 11 arranged in the original convey path, mirrors 10b, 10c and 10d for reflecting the illumination light, a lens 10e for condensing the reflected light, and an image reading element 10f such as a CCD. Further, a pair of discharge rollers 12 serve to discharge the original G the image of which was read onto a discharge tray 13. Incidentally, convey guides 14 for guiding the original G are disposed between the respective pairs of rollers. Now, a relation between the lengths shown in Fig. 1 will be explained. A convey length La 1 between the registration means 8 and the second pair of convey rollers 6 has a relation (La 1 < Ls 1 ) with respect to the length Ls 1 of the original G in the original conveying direction. Further, a convey length La 2 between the registration means 8 and the first pair of convey rollers 5 has a relation (La 2 < Ls 2 ) with respect to the length Ls 2 of the original G in the original conveying direction. Further, lengths obtained by adding a convey length Lb between the registration means 8 and the sensor 9 of light permeable type to the convey lengths La 1 , La 2 , respectively have relations {(La 1 + Lb) > Ls 1 }, {(La 2 + Lb) > Ls 2 } with respect to the lengths Ls 1 , Ls 2 of the original G in the original conveying direction, respectively. Next, an operation of the original image reading apparatus having the above-mentioned construction will be explained. When the originals G are set on the original stacking plate 1, the lengths Ls 1 , Ls 2 of the originals G in the original conveying direction are detected by the sensors 2a, 2b. If the detected length of the original is Ls 1 , since the length Ls 1 is greater than the convey length La 1 between the registration means 8 and the second pair of convey rollers 6 (La 1 < Ls 1 ) and is smaller than the length (La 1 + Lb) obtained by adding the convey length Lb between the registration means 8 and the sensor 9 of light permeable type to the convey length La 1 {(La 1 + Lb) > Ls 1 }, the second pair of convey rollers 6 previously separated from each other are abutted against each other. On the other hand, if the detected length of the original is Ls 2 , since the length Ls 2 is greater than the convey length La 1 between the registration means 8 and the second pair of convey rollers 6 (La 1 < Ls 2 ) and is also greater than the length (La 1 + Lb) obtained by adding the convey length Lb between the registration means 8 and the sensor 9 of light permeable type to the convey length La 1 {(La 1 + Lb) < Ls 2 }, the second pair of convey rollers 6 is still maintained in the separated condition. After the abutment/separation of the second pair of convey rollers 6 is effected, the pick-up roller 3 is rotated to feed out uppermost original(s) on the original stacking plate 1 toward the pair of separation rollers 4. If a single original is fed to the pair of separation rollers 4, the latter sends the original toward the first pair of convey rollers 5; whereas, if two or more originals are fed to the pair of separation rollers 4 at a time, the lower roller 4B of the pair of separation rollers 4 is rotated in a direction opposite to the original conveying direction, thereby separating the uppermost original alone from the other original(s), and the separated original is sent toward the first pair of convey rollers 5. However, since such a separation mechanism does not directly relate to the present invention, detailed explanation thereof will be omitted. If the length of the original supplied in this way is Ls 1 , since the second pair of convey rollers 6 are abutted against each other, the original is sent to the pairs of regist rollers 8a, 8b along the convey guides 14 by means of the first and second pairs of convey rollers 5, 6; whereas, if the length of the original is Ls 2 , since the second pair of convey rollers 6 are separated from each other, the original is sent to the pairs of regist rollers 8a, 8b along the convey guides 14 by means of the first pair of convey rollers 5. The pairs of regist rollers 8a, 8b are rotated at the same speed as the convey speeds of the pairs of convey rollers 5, 6. As shown in Fig. 3, the original pinched between the pairs of regist rollers 8a, 8b is conveyed toward the transparent glass 11 (forming a part of an image reading portion); meanwhile, the original passes through the sensors 9a, 9b of light permeable type. As soon as the original passes through the sensors 9a, 9b, signals are emitted from the respective sensor, and inclination of the tip end of the original is calculated on the basis of these signals by means of a calculation circuit (not shown). The rotations of the respective drive motors M 1 , M 2 for the pairs of regist rollers 8a, 8b are controlled by the control means (not shown) on the basis of the calculated inclination, thereby correcting the skew-feed of the original G. Next, the skew-feed correction control will be explained with reference to a block diagram shown in Fig. 4 and a flow chart shown in Fig. 5. When the tip end of the original passes through the sensors 9a, 9b of light permeable type, due to a time difference between output timings of both sensors, count data corresponding to the skew amount is outputted from a counter circuit 21. In a timing chart shown in Fig. 6, the sensor 9a is turned ON before the sensor 9b is turned ON. The counter circuit 21 is started when the sensor output of the sensor 9a is changed to a high level, and a counting operation is continued until the sensor output of the sensor 9b is changed to a high level. If the sensor 9b is turned ON before the sensor 9a is turned ON, the counter circuit 21 is operated similarly. Then, when both sensor outputs of the sensors 9a, 9b become the high levels, the counter circuit 21 emits an interruption signal which is in turn sent to a microcomputer 22 (step S1 in Fig. 5). When the interruption signal is inputted to the microcomputer 22, the latter reads in the count data, thereby determining the amount and direction of the skew-feed of the original (step S2). In the illustrated case, since the original passes through the sensor 9a prior to the sensor 9b, the microcomputer 22 controls reduction of motor M 1 for driving the regist rollers 8a nearer to a preceding corner 9a (among both corners 9a, 9b) of the tip end of the original G via step motor driver 23, thereby correcting the skew-feed of the original (step S3). The reduction control will be fully described later with reference to a relation between drive frequency and control time shown in Fig. 7. The drive frequency is clocks outputted to the motor drive for the drive motor M 1 . When the skew-feed correction control is not effected, the drive frequency normally has a value of 5000 Hz. When the reduction control (step S3) is started, the speed of the drive motor M 1 is gradually reduced. In the microcomputer 22, the difference in clock outputs outputted to the motors M 1 , M 2 is counted by a counter circuit (not shown) provided in the microcomputer (step S4). When the counted value becomes a half (1/2) of a count amount corresponding to the skew correction amount (i.e., when the correction of the half of the skew amount is finished), the acceleration control is started at a point corresponding to a return point 1 shown in Fig. 7 (step S5). Similar to the reduction control, the difference in clock outputs outputted to the motors M 1 , M 2 is counted by the counter circuit (not shown) provided in the microcomputer, and, when the counted value becomes equal to the count amount corresponding to the skew correction amount, the skew-feed correction is finished (corresponding to "control finish 1" in Fig. 7) (step S6). That is to say, a succeeding corner 9b is advanced more than the preceding corner 9a by an amount corresponding to the delayed distance. If the skew amount is great, the time period for the reduction control is increased (corresponding to "return point 2" in Fig. 7) and the time period for the acceleration control is also increased accordingly (corresponding to "control finish 2" in Fig. 7). In this case, since the trailing end of the original G is greatly rotated, if the trailing end of the original is pinched between the pair of convey rollers 5 or 6, the rotational movement of the original will be limited. However, in the illustrated embodiment, as mentioned above, when the tip end of the original reaches the sensors 9a, 9b, the trailing end of the original having the length of Ls 1 always leaves the pair of convey rollers 6, or the trailing end of the original having the length of Ls 2 leaves the pair of convey rollers 5 and the pair of convey rollers 6 are previously separated from each other. Accordingly, when the skew-feed of the original is corrected by the pairs of regist rollers 8a, 8b, the rotational movement of the trailing end of the original is not restrained. Thus, the skew-feed of the original can be corrected with high accuracy regardless of the length of the original. Further, according to the illustrated embodiment, since the abutment or separation of the pair of convey rollers 6 can be selected by previously detecting the size of the original (or by inputting the size data by the operator), the timing control of the abutting and separating mechanism 7 for abutting or separating the pair of convey rollers 6 is not required to be accurately performed. Further, when the originals are continuously supplied, since the abutting or separating operation for the pair of convey rollers 6 is not performed every time, the service life of the abutting and separating mechanism 7 is not severely selected. Further, there is no bad influence upon the original reading due to any shock generated in the abutting or separating operation of the pair of convey rollers 6. (Second Embodiment which is not part of the present invention) Next, a sheet conveying apparatus according to a second embodiment which is not part of the present invention will be explained. Incidentally, also in this second embodiment, a sheet conveying apparatus is applied to an original image reading apparatus, and the same functional elements are designated by the same reference numerals and explanation thereof will be omitted. Fig. 8 is a schematic sectional view of an original conveying apparatus according to the second embodiment and an associated image reading apparatus. In Fig. 8, a sensor (sheet end detection means) 15 for detecting a tip end of an original G is arranged in front of the registration means 8. In this embodiment, abutment and separation of a second pair of convey rollers 6 are effected by an abutting and separating mechanism 7 shown in Fig. 2B, and the second pair of convey rollers 6 are normally in a separated condition. The abutting and separating mechanism 7 shown in Fig. 2B comprises a motor 7d a rotation of which is controlled by a control means (not shown), a motor gear 7e supported by a shaft of the motor 7d, and a cam 7f having a partial toothed portion meshed with the motor gear 7e. The cam 7f is rotatably supported at its axis corresponding to a center of curvature of the toothed portion and has a lever portion adapted to be engaged by a shaft of the roller 6a. In Fig. 8, the original G fed out by the pick-up roller 3 and separated by the pair of separation rollers 4 is sent to the registration means 8 by means of the pairs of convey rollers 5, 6. Meanwhile, the tip end of the original G is detected by the sensor 15 immediately before the tip end reaches the registration means 8. The pair of separable convey rollers 6 are separated from each other at a predetermined timing (i.e., within a time period from when the tip end of the original G is detected by the sensor 15 to when the tip end is pinched between the registration means 8). That is to say, in the abutting and separating mechanism 7 shown in Fig. 2B, when the motor 7d connected to the motor gear 7e is rotated in an anti-clockwise direction in Fig. 2 by a predetermined angle to rotate the cam 7f meshed with the motor gear 7e in an anti-clockwise direction, the lever portion of the cam 7f is engaged by the shaft of the roller 6a to shift the roller 6a away from the roller 6b, thereby separating the pair of convey rollers 6 from each other. In the condition that the pair of convey rollers 6 are separated from each other, the original G is conveyed by the registration means 8 to pass through the sensor 9 (sheet skew amount detection means), where the skew amount of the original is detected. As is in the first embodiment, the rotations of the drive motors of the registration means 8 are controlled on the basis of the detected skew amount, thereby correcting the skew-feed of the original. In the skew-feed correcting operation, since the pair of convey rollers 6 are separated from each other, the trailing end of the original G is not restrained in the convey guides 14 regardless of the length of the original in the original conveying direction, so that the skew-feed of the original can be corrected by the registration means 8 with high accuracy. According to the second embodiment, since the abutment and separation of the pair of convey rollers 6 are effected on the basis of the signal emitted from the sensor 15 arranged in front of the registration means 8, the skew-feed of the original can be corrected with high accuracy regardless of the length of the original in the original conveying direction. (Third Embodiment) Next, a sheet conveying apparatus according to a third embodiment of the present invention will be explained with reference to the accompanying drawings. Incidentally, in this third embodiment, a sheet conveying apparatus is applied to an original image reading apparatus, and the same functional elements are designated by the same reference numerals and explanation thereof will be omitted. Fig. 9 is a schematic sectional view of an original conveying apparatus according to the third embodiment and an associated image reading apparatus, and Fig. 10 is a perspective view showing a shifting mechanism for shifting a second pair of convey rollers. In Fig. 9, a sensor array (sheet length detection means) 16 for detecting a length of an original G on the original stacking plate 1 is constituted by sensors of reflection type arranged side by side along the original conveying direction. A second pair of convey rollers 17 are designed so that they can be shifted in an up-and-down direction along the original convey path. In Fig. 10, a shifting mechanism 18 for shifting the second pair of convey rollers 17 comprises a support plate 18a for rotatably supporting the paired rollers 17, guide rods 18b, 18c for guiding the support plate 18a, a timing belt 18d to which one end of the support plate 18a is secured, pulleys 18e, 18f around which the timing belt 18d is mounted, and a drive motor 18g connected to the pulley 18f and acting as a drive source for this pulley. The mechanism 18 is drivingly controlled by a control means (not shown). In Fig. 9, the length (in the original conveying direction) of the original G rested on the original stacking plate 1 is detected by the sensor array 16. On the basis of a detection signal from the sensor array, the second pair of convey rollers 17 are shifted, by the shifting mechanism 18, to a position where a convey distance La between the second pair of convey rollers 17 and the registration means 8 becomes smaller than the length Ls of the original in the original conveying direction and a convey distance (La + Lb) between the second pair of convey rollers 17 and the sensor (skew amount detection means) 9 becomes greater than the length Ls of the original in the original conveying direction {La < Ls < (La + Lb)}. Accordingly, the original G is conveyed by the second pair of conveys rollers 17 until it is pinched between the registration means 8, and, when the tip end of the original reaches the sensor 9, the trailing end of the original leaves the second pair of convey rollers 17. Thus, when the skew-feed correction is effected, the original is pinched only by the registration means 8. According to this embodiment, since the pair of convey rollers 17 can be shifted along the original convey path, even when the length of the original in the original conveying direction is changed, the skew-feed of the original can be corrected with high accuracy. (The Other Embodiments) In the first and third embodiments, while an example that the sensors of reflection type are arranged on the original stacking plate as the sheet length detection means for detecting the length in the original conveying direction) of the original rested on the original stacking plate was explained, the present invention is not limited to such an example. For example, the length of the original in the original conveying direction may be detected by a sheet length detection means 19 as shown in Figs. 11A and 11B on the basis of movement of the original on the original stacking plate. The sheet length detection means 19 shown in Figs. 11A and 11B comprises a driven roller 19a having an outer surface coated by elastic material such as rubber having relatively high friction of coefficient and driven by movement of the uppermost original G, a circular timing disc 19c having a plurality of slits arranged along a circle coaxial with a shaft 19b of the driven roller 19a, and a photo-interrupter 19d of permeable type arranged to sandwitch one of slits of the timing disc 19c. Incidentally, in Fig. 11A, the reference numeral 3 denotes a pick-up roller, and 4 denotes a pair of separation rollers. In Figs. 11A and 11B, the uppermost originals G are fed out by the pick-up roller 3 and are separated one by one by means of the pair of separation rollers 4. The separated original is further conveyed in the original conveying direction. When the uppermost original G is moved in this way, the driven roller 19a of the sheet length detection means 19 contacted with the uppermost original G is rotated in a direction shown by the arrow in Fig. 11A, thereby rotating the timing disc 19c. In this case, since the slits of the timing disc 19c are intermittently passed through between light emitting and light receiving portions of the photo-interrupter 19d to repeatedly prevent and permit the passage of light, ON/OFF signals as shown in Fig. 12 are regularly generated. The moving amount of the original is detected by the number of these signals. When the trailing end of the uppermost original passes through the roller 19a, since a next original is still stopped till a next original supplying operation, the roller 19a is not rotated, with the result that the photo-interrupter 19d continues to emit the ON signal or the OFF signal. Accordingly, the length of the original in the original conveying direction can be detected on the basis of a time period during which the signals are successively generated while the original is being shifted. By using the sheet length detection means as mentioned above, since the length of each original which is being supplied can be detected, even when the originals rested on the original stacking plate have different lengths, the abutment or separation of the second pair of convey rollers or the shifting movement of the second pair of convey rollers can be effected correctly, thereby correcting the skew-feed of the original with high accuracy. In the first embodiment, while an example that the second pair of convey rollers are normally in the separated condition was explained, the present invention is not limited to such an example. For example, the second pair of convey rollers may be normally in the abutted condition, and such rollers may be separated from each other on the basis of the detection signal regarding the length of the original in the original conveying direction. In this case, the same advantage as that of the first embodiment can be achieved. Further, this is true also in the second embodiment. Furthermore, in the aforementioned embodiments, while an example that only one pair of convey rollers among two pairs of convey rollers can be abutted against or separated from each other was explained, the present invention is not limited to such an example. For example, a plurality of pairs of convey rollers may be abutted against or separated from each other. Further, in the aforementioned embodiments, while an example that the sheet conveying apparatus is applied to the original image reading apparatus was explained, the present invention is not limited to such an example. For example, it should be noted that the sheet conveying apparatus according to the present invention may be applied to any image forming apparatuses. In an example of the image forming apparatus, for example, in Fig. 1, recording materials such as copy sheets are stacked or rested on the original stacking plate (sheet stacking plate) in place of the originals G, and a recording system (including a recording head, a head driver circuit and the like) is provided in place of the optical system 10a to 10f, 11. In this case, if a copy sheet cassette is used, size information from the copy sheet cassette will be used as a means for determining a sheet length. Furthermore, by combining the original image reading apparatus and the image forming apparatus having the feature of the present invention, an image forming apparatus wherein image information read from the original is recorded on the copy sheet may be provided. In the aforementioned embodiments, while an example that, when the output of the sensor 9a reaches the high level prior to the sensor 9b, the reduction control of the motor M 1 is effected to correct the skew-feed of the original was explained, the skew-feed of the original may be corrected by temporarily stopping the motor M 1 . Alternatively, the skew-feed of the original may be corrected by effecting the acceleration control of the motor M 2 . In addition, the reduction control of the motor M 1 and the acceleration control of the motor M 2 may be effected simultaneously. Further, in the aforementioned embodiments, while an example that the reduction control or acceleration control of the pulse motor is effected by changing the speed of the motor at a constant rate was explained, the present invention is not limited to such an example. For example, the speed of the motor may be changes steppingly from a normal speed V 0 to a higher speed V H or to a lower speed V L . Further, in the aforementioned embodiments, while an example that the greater the skew amount of the original, the longer the time period of the reduction control to increase the difference in speed between two convey means was explained, the present invention is not limited to such an example. For example, the time period of the reduction control may be kept constant and, as the skew amount of the original is increased, the reduction rate may be increased or decreased. Incidentally, the above-mentioned sheet conveying apparatuses can be applied to image forming apparatuses, as well as the original image reading apparatuses. Particularly, when the sheet conveying apparatus is arranged at an upstream side of an image forming means for forming an image on a sheet, the image can be formed on the sheet at a correct position. Next, an embodiment wherein the registration of the sheet can be effected with high accuracy even after an apparatus was used for a long time or used repeatedly will be explained with reference to Fig. 13. In Fig. 13, additional two sets (15a, 15b) of sensors 15 of light permeable type are arranged between the sensor 9 of light permeable type for detecting the skew amount of the original and the image reading portion 11. When the skew-feed correction is finished before the tip end reaches the sensors 15, the result of the skew-feed correction can be ascertained by the sensor 15, and, thus, it is not required to measure the skew amount of the original again at the image reading portion 11. Normally, the measurement of the skew amount at the image reading portion requires the provision of counters arranged along a main scan direction (reading line direction). However, when the sensors 15 of light permeable type are added, the provision of the counters is not required. By using such sensors 9, 15 of light permeable type, the skew-feed of the original can be effected permanently with high accuracy. Further, in this embodiment, when there are adequate time period and distance between the sensors 15 and the image reading portion 11, the fine correction of the skew-feed of the original can be effected between the sensors 15 and the image reading portion 11, if necessary, thereby completing the correction of the skew-feed more accurately. However, since the pairs of regist rollers 8a, 8b are gradually worn to gradually decrease the friction force of each roller, the original convey speed of the regist rollers is also gradually decreased. As a result, the accuracy of the skew-feed correction (which was perfect initially) is also worsened gradually (but very slowly). In consideration of the above, the skew amount of the original is measured again at the image reading portion by counting a different in time between the output timings of the sensors 15a, 15b, and the measured amount is stored in a memory 25 (Fig. 14). After a certain number of originals (for example, several hundred thousands) were treated, if an average value of the skew amount (after skew-feed correction) regarding the latest ten thousand originals is not zero (0 mm) but is offset toward either the pair of regist rollers 8a or the pair of regist rollers 8b (for example, if the tip end corner of the original near the pair of regist rollers 8a advances by 0.1 mm more than the tip end corner of the original near the pair of regist rollers 8b), when the next correction is effected, the speed of the pair of regist rollers 8a is delayed by 0.1 mm in comparison with the pair of regist rollers 8b. Such correction control is shown in a flow chart of Fig. 15. A count data (skew amount) obtained by counting the difference in time between the output timings of the sensors 9a, 9b is read (step S12). Then, the average value of the skew amount (after skew-feed correction) regarding the latest ten thousand originals is added to the read count data (step S17), and then the reduction control is effected to perform the correction of the skew amount after addition (step S13). Then, after the treatments similar to the flow chart shown in Fig. 5 are completed (steps S14 to S16), the correction is ended. Thereafter, the image reading operation may be started, or the skew-feed correction may be effected again on the basis of the skew amount (after skew-feed correction) calculated by the microcomputer 22 on the basis of the difference in time between the detection signals from the sensors 15a, 15b, before the image reading operations is started. Such skew-feed correction is effected on the basis of the flow chart shown in Fig. 5. In the step S12, the skew amount (after skew-feed correction) calculated on the basis of the outputs of the sensors 15a, 15b is read in. By feedback-adjusting such correction amount on the basis of the latest corrected result, the control can be effected permanently with high accuracy. Further, even if the outer diameters of the regist rollers 8 are relatively greatly decreased due to the wear or other abnormity, since the self-feedback control can be performed, the frequency of exchange of the rollers by an expert service man can be greatly reduced. As mentioned above, in the conveying apparatus wherein the skew-feed is corrected while conveying the sheet, since the abutment or separation, or the shifting movement of the paired rollers in the sheet convey means is controlled on the basis of the detection signal regarding the sheet length in the sheet conveying direction or the trailing end of the sheet and since, when the skew-feed of the sheet is corrected by using the registration means, the trailing end of the sheet in the sheet convey path is released from the paired rollers (without being pinched by said rollers), the sheets can be continuously conveyed with minimum sheet interval while correcting the skew-feed of the sheet with high accuracy. Further, by applying the sheet conveying apparatus to the image reading apparatus or the image forming apparatus, when the image reading apparatus or the image forming apparatus is operated at the same speed, the number of sheets to be treated per unit time can be increased, thereby improving the productivity. Further, as mentioned above, in the conveying apparatus wherein the skew-feed is corrected while conveying the sheet, since after the skew-feed of the sheet was corrected by the registration means, the result of the skew-feed correction is measured and stored and the measured data can be fed-back, the correction accuracy can be maintained and improved permanently. The present invention relates to a sheet conveying apparatus wherein a pair convey rollers arranged upstream of a registration means for correcting a skew-feed of a sheet in a sheet conveying direction can be abutted against or separated from each other, or can be shifted in accordance with a length of the sheet in the sheet conveying direction. As the result, when the skew-feed of the sheet is corrected by the registration means, a trailing end of the sheet is not restrained.

Claims: A sheet conveying apparatus comprising: a registration means (8) for correcting a skew-feed of a sheet; an abuttable/separable sheet convey means (6) arranged at an upstream side of said registration means (8) in a sheet conveying direction; a sheet length detection means (2a, 2b); a sheet skew amount detection means (9) characterized in that said sheet skew amount detection means (9) is arranged downstream of said registration means in the sheet conveying direction; said sheet convey means (6) are abuttable/separable in accordance with the length of the sheet previously detected by said sheet length detection means (2a, 2b) and in that when a sheet length in the sheet conveying direction detected by said sheet length detection means (2a, 2b) is greater than a convey length (L a 1) between said sheet convey means (6) and said registration means (8), and smaller than a convey length (L a1 +L b ) between said sheet convey means (6) and said sheet skew amount detection means (9), said sheet convey means (6) are abutted against each other. Sheet conveying apparatus according to claim 1, characterized in that when a sheet length in the sheet conveying direction is greater than a convey length (L a1 ) between said sheet convey means (6) and said registration means (8), and greater than a convey length (L a1 +L b ) between said sheet convey means (6) and said sheet skew amount detection means (9), said sheet convey means (6) are separated from each other. Sheet conveying apparatus according to claim 1 or 2, characterized in that said sheet length detection means (2a, 2b) arranged at an upstream side of said sheet convey means (6) is a sheet end detection means. Sheet conveying apparatus according to one of claims 1 to 3, characterized in that said sheet convey means (6) includes a pair of rollers (6a, 6b), and one (6a) of said pair of rollers can be abutted against or separated from the other roller (6b). Sheet conveying apparatus, comprising: a registration means (8) for correcting a skew-feed of a sheet, arranged upstream of an image forming portion or an image reading portion in a sheet conveying direction; a sheet convey means (6) arranged upstream of said registration means (8) in the sheet conveying direction; a sheet length detection means (2a, 2b); a sheet skew amount detection means (9) arranged downstream of said registration means in the sheet conveying direction; characterized in that said sheet convey means (6) is shiftable in accordance with the length of the sheet previously detected by said sheet length detection means (2a, 2b), to a position where a convey length (L a1 ) between said sheet convey means (6) and said registration means (8) is smaller than a sheet length in the conveying direction, and a convey length (L a1 +L b ) between said sheet convey means (6) and said sheet skew amount detection means (9) is greater than the sheet length in the sheet conveying direction. Image forming apparatus comprising a sheet conveying apparatus according to one of claims 1 to 5. Image reading apparatus comprising a sheet conveying apparatus according to one of claims 1 to 5. Image forming apparatus comprising an image reading apparatus according to claim 7. A sheet conveying apparatus according to claim 1 or 5, wherein said sheet length detection means is arranged at an upstream side of said sheet convey means. A sheet conveying apparatus according to claim 1 or 5, wherein said sheet length detection means determines a sheet length based on sheet size information inputted by an operator. A sheet conveying apparatus according to claim 1 or 5, wherein said sheet length detection means determines a sheet length based on sheet size information from a copy sheet cassette.

Inventors: MARUYAMA HIROYOSHI, YASHIRO MASAHIKO, MARUYAMA, HIROYOSHI, YASHIRO, MASAHIKO, MARUYAMA, HIROYOSHI, C/O CANON KABUSHIKI KAISHA, YASHIRO, MASAHIKO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: B65H9
IPC codes: B65H5, B65H7, B65H9